Explore detached and floating UI elements

[draganescu]

I would like to raise the utility of having floating detachable UI elements, such as the the header, block toolbar even when fixed, the sidebars (primaty and secondary) and the breadcrumbs.

For a 1st iteration we could try only floating detached UI, leaving the possibility for repositioning detached UI for later. As a good reference point for this is the new evolution of Figma's UI where the chrome of the application is detached and floating on top of the workspace.

This interface architecture would allow us:

• Removing the float and reflow effects of simply opening or closing a panel. On smaller viewports it becomes impossible to imagine what the result is without previewing the rendered page, which is completely against a basic proposal of the block editor.
• Allowing a better use of distraction free and focus modes, for power usecases, opening the entire canvas to design ideation
• Future improve customisability of the UI - for example, list view on the right, inspector on the left